Hi Ed, (01)
I said: (02)
>> If you treat RDF and RDFS as simple modelling syntaxes (that's all they
>> are), you can use them quite nicely to represent your ontology - even an
>> extensional, 4D, higher-order ontology. It's less easy to do that with
OWL,
>> as that language tries to be a bit more clever (and as always with things
>> that try to be too clever, shoots itself in the foot). It's just a syntax
-
>> the semantics should be in your ontology, not in the language you choose
to
>> publish it in. (03)
You said: (04)
This is false, and it misses an important point. (05)
> RDF introduces a minimal modeling vocabulary, which allows the
> expression of essentially arbitrary relations and logical statements.
> OWL/DL introduces a much richer vocabulary, but removes certain RDF
> vocabulary elements that allow the expression of arbitrary logical
> statements (e.g., "implies"). What that does is to limit the kinds of
> statements you can make in OWL. (And those limitations guarantee that
> certain reasoning algorithms will terminate in boundable time, which is
> not true of RDF.) In each case, the base vocabulary has a (strongly)
> specified semantics and that semantics enables the interpretation of
> sentences in the language. It is not "just a syntax".
<SNIP> (06)
I did say this requires a bit of imagination, or at least a bit of patience
with my somewhat slapdash description. I'll try to clarify... (07)
When I say semantics, I am referring to stuff in the real world. When a
computer scientist says "semantics" they are usually, as far as I can see,
referring to structure of data. They get the two things mixed up a lot, but
that's probably because they've played too much World of Warcraft. There are
no REAL WORLD SEMANTICS in RDF and RDFS, and probably not in OWL either.
None of these languages has criteria for extent (at least not real-world
extent), so I am free to use them to describe what I choose, and have done
so with gay abandon. I get some benefit from descending my type from
RDFS:class because there is some set-theoretic stuff in there I can re-use.
But...and this is important...being set-theoretic doesn't make it semantic.
I can have nonsense sets that refer to nothing in the real world. (08)
If you're prepared to play fast and loose with RDF and RDFS, you can do
things like having first-order representations of higher-order ontologies -
i.e. you don't use the rdf:Type for types of types, you simply use a new rdf
property for that. Silly, I know, but the inference wizards of Warcraft like
everything to be nice and flat. Something to do with finite computation
time...though taking 24hrs to make an inference with all the deductive power
of a red setter *is* acceptable in flat world. Not sure it'll ever fly in
business, mind you. (09)
None of this should be a surprise. ISO15926 is a higher order, 4D ontology
represented by EXPRESS (a distinctly first-order language). I read Barry
Smith's comments on how this couldn't possibly be an ontology, and I think
Matthew responded to that far more eloquently than I could. I will say this
though - it's really easy to confuse the representation of the ontology with
the ontology itself. If I decide I'm going to tattoo my ontology in barcode
on my left buttock, that is also an acceptable approach (technically I mean,
not socially) provided I have documented how the barcode/arse-cheek
combination maps onto the real world. This is what we do when we profile UML
for IDEAS (as you correctly pointed out). What I think you missed is that
you can do almost exactly the same thing with RDFS. The first thing we did
was create instances of rdfs:Class for all our IDEAS ontic categories. From
that point on, we don't use any of the RDF elements in our encoding. We may
or may not subtype our type-instance relationship from rdf:type (depends if
we want to keep the level five orcs happy) and we do subtype ideas:Type from
rdfs:Class, but this is an engineering decision to allow us to leverage the
wealth of open-source stuff that's out there. (010)
Cheers
--
Ian (011)
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